Various United States patents have been obtained for hand-operated backpack-type sprayer combinations adapted to spray insecticides, fertilizers, fungicides, etc. These include: U.S. Pat. Nos. 4,690,331, 4,702,416, 4,702,419, 4,768,714, and 4,798,333. The constructions shown and described in the specified patents include relatively large-diameter diaphragms, and these are often combined with pistons in such manner that the diaphragm-piston combinations effect not only pumping of the liquid but also agitation of the liquid in the bottom of the back-carried container.
The diaphragms taught by the above-specified patents are relatively large in diameter and are often employed in combination with relatively large-diameter nuts. Thus, there are size and expense requirements that are not fully satisfied by the indicated prior-art constructions.
Another important consideration is that it is desirable to achieve even greater protection against eventual leakage than is afforded by the indicated prior-art construction. Diaphragms are very good sealing elements against leakage. However, after long-continued use in association with certain types of chemicals, even the best diaphragms may eventually crack. Upon occurrence of such an event, the insecticide or fertilizer would tend to leak out the bottom of the structure and may contact the operator, or may contact some location where the presence of the liquid is not desired.